docs: def bootstrapify(self):
          """Add bootstrap cdn to headers of html"""
          if self.__htmltree is None:
              #raise Exception("HtmlTree has not been made yet")
              self.__make_tree()
  
          # add bootstrap cdn to head
          self.__htmltree.find('head').append(
              E.LINK(rel="stylesheet",
                     href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.1/css/bootstrap.min.css",
                     type="text/css"))
  
          # center images
          for img_parent in self.__htmltree.xpath("//img/.."):
              # the space before the class to insert is CRITICAL!
              img_parent.attrib["class"] += " text-center"
  
          # make images responsive
          for img in self.__htmltree.xpath("//img"):
              # the space before the class to insert is CRITICAL!
              img.attrib["class"] += " img-responsive"